[BUGFIX] Make sure XML parser is created everytime when needed

Currently, EM's MirrorXmlPushParser, which is based on ext/xml,
is not properly working. The constructor creates the parser and
within parseXML() method the parser resource is freed.
However, when clicking on retrieve/update button method
parseXML() is invoked multiple times which makes subsequent calls
fail.

Creating the parser is now done within parseXML() method.

Resolves: #25292
Releases: 6.1, 6.0, 4.7, 4.5
Change-Id: I7c6c15f65d08fb0264d7d66cc584cbe7ff423b45
Reviewed-on: https://review.typo3.org/18965
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn